This invention relates to elongate, closed-shell air depolarized electrochemical cells. This invention is related specifically to metal-air air depolarized electrochemical cells illustrated herein as elongate cylindrical cells, and is described herein in relation to cells having the size generally known as "AA."
The advantages of air depolarized cells have been known as far back as the 19th century. Generally, an air depolarized cell draws oxygen from air of the ambient environment, for use as the cathode active material. Because the cathode active material need not be carried in the cell, the space in the cell that would have otherwise been required for carrying cathode active material can, in general, be utilized for containing anode active material.
Accordingly, the amount of anode active material which can be contained in an air depolarized cell is generally significantly greater than the amount of anode active material which can be contained in a 2-electrode cell of the same overall size. By "2-electrode" cell, we mean an electrochemical cell wherein the entire charge of both anode active material and cathode active material are contained inside the cell structure when the cell is received by the consumer.
Generally, for a given cell size, and similar mass, an air depolarized cell can provide a significantly greater number of watt-hours of electromotive force than can a similarly sized, and similar mass, 2-electrode cell using the same, or a similar, material as the anode electroactive material.
Several attempts have been made to develop and market commercial applications of metal-air cells. However, until about the 1970's, such cells were prone to leakage, and other types of failure.
In the 1970's, metal-air button cells were successfully introduced for use in hearing aids, as replacement for 2-electrode cells. The cells so introduced were generally reliable, and the incidence of leakage had generally been controlled sufficient to make such cells commercially acceptable.
By the mid 1980's, zinc-air cells became the standard for hearing aid use. Since that time, significant effort has been made toward improving metal-air hearing aid cells. Such effort has been directed toward a number of issues common to all manufacturers of such cells. For example, efforts have been directed toward increasing electrochemical capacity of the cell, toward consistency of performance from cell to cell, toward control of electrolyte leakage, toward providing higher voltages desired for newer hearing aid technology, toward higher limiting current, and toward controlling movement of moisture into and out of the cell, and the like.
While metal-air button cells have found wide-spread use in hearing appliances, air depolarized cells have not had wide-spread commercial application for any other end uses, or in other than small button cell sizes.
The air depolarized button cells readily available as items of commerce are generally limited to sizes of no more than 0.6 cm.sup.3 overall volume. In view of the superior ratio of "watt-hour capacity/mass" of air depolarized cells, it would be desirable to provide air depolarized electrochemical cells for other applications. It would especially be desirable to provide air depolarized electrochemical cells which are relatively much larger than button cells. For example, it would be desirable to provide such cells in "AA" size.
It is an object of the invention to provide an air depolarized cell which is relatively larger than a hearing aid button cell and which has a greater overall discharge cycle capacity than a similarly-sized alkaline manganese dioxide cell.
It is another object to provide an air depolarized cell which is relatively larger than a hearing aid button cell, which has an overall discharge capacity at least as great as a similarly-sized alkaline manganese dioxide cell, and wherein the energy/mass ratio of such cell is significantly greater than the energy/mass ratio of a similarly-sized alkaline manganese dioxide cell.